Which characters are valid in CSS class names/selectors?

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What characters/symbols are allowed within CSS class selectors? I know that the following characters are invalid, but what characters are valid?

~ ! @ $ % ^ & * ( ) + = , . / ' ; : " ? > < [ ] \ { } | ` # 
This Question Has 9 Answeres | Orginal Question | Darryl Hein

Basically every characters accepted, expect white space, all Unicode characters are also allowed... so you can use Numbers or Characters, but at the same time, always try to follow best practice naming for your CSS! Messy, not meaningful naming for CSS can create a big mess in your code structure...

You can check directly at the CSS grammar.

Basically1, a name must begin with an underscore (_), a hyphen (-), or a letter(az), followed by any number of hyphens, underscores, letters, or numbers. There is a catch: if the first character is a hyphen, the second character must2 be a letter or underscore, and the name must be at least 2 characters long.

-?[_a-zA-Z]+[_a-zA-Z0-9-]* 

Identifiers beginning with a hyphen or underscore are typically reserved for browser-specific extensions, as in -moz-opacity.

1 It's all made a bit more complicated by the inclusion of escaped unicode characters (that no one really uses).

2 Note that, according to the grammar I linked, a rule starting with TWO hyphens, e.g. --indent1, is invalid. However, I'm pretty sure I've seen this in practice.

For HTML5/CSS3 classes and IDs can start with numbers.

The complete regular expression is:

-?(?:[_a-z]|[\200-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])(?:[_a-z0-9-]|[\200-\377]|\\[0-9a-f]{1,6}(\r\n|[ \t\r\n\f])?|\\[^\r\n\f0-9a-f])* 

So all of your listed character except “-” and “_” are not allowed if used directly. But you can encode them using a backslash foo\~bar or using the unicode notation foo\7E bar.

To my surprise most answers here are wrong. It turns out that:

Any character except NUL is allowed in CSS class names in CSS. (If CSS contains NUL (escaped or not), the result is undefined. [CSS-characters])

Mathias Bynens’ answer links to explanation and demos showing how to use these names. Written down in CSS code, a class name may need escaping, but that doesn’t change the class name. E.g. an unnecessarily over-escaped representation will look different from other representations of that name, but it still refers to the same class name.

Most other (programming) languages don’t have that concept of escaping variable names (“identifiers”), so all representations of a variable have to look the same. This is not the case in CSS.

Note that in HTML there is no way to include space characters (space, tab, line feed, form feed and carriage return) in a class name attribute, because they already separate classes from each other.

So, if you need to turn a random string into a CSS class name: take care of NUL and space, and escape (accordingly for CSS or HTML). Done.

Read the W3C spec. (this is CSS 2.1, find the appropriate version for your assumption of browsers)

edit: relevant paragraph follows:

In CSS, identifiers (including element names, classes, and IDs in selectors) can contain only the characters [a-z0-9] and ISO 10646 characters U+00A1 and higher, plus the hyphen (-) and the underscore (_); they cannot start with a digit, or a hyphen followed by a digit. Identifiers can also contain escaped characters and any ISO 10646 character as a numeric code (see next item). For instance, the identifier "B&W?" may be written as "B\&W\?" or "B\26 W\3F".

edit 2: as @mipadi points out in Triptych's answer, there's this caveat, also in the same webpage:

In CSS, identifiers may begin with '-' (dash) or '_' (underscore). Keywords and property names beginning with '-' or '_' are reserved for vendor-specific extensions. Such vendor-specific extensions should have one of the following formats:

'-' + vendor identifier + '-' + meaningful name '_' + vendor identifier + '-' + meaningful name 

Example(s):

For example, if XYZ organization added a property to describe the color of the border on the East side of the display, they might call it -xyz-border-east-color.

Other known examples:

 -moz-box-sizing -moz-border-radius -wap-accesskey 

An initial dash or underscore is guaranteed never to be used in a property or keyword by any current or future level of CSS. Thus typical CSS implementations may not recognize such properties and may ignore them according to the rules for handling parsing errors. However, because the initial dash or underscore is part of the grammar, CSS 2.1 implementers should always be able to use a CSS-conforming parser, whether or not they support any vendor-specific extensions.

Authors should avoid vendor-specific extensions

My understanding is that the underscore is technically valid. Check out:

https://developer.mozilla.org/en/underscores_in_class_and_id_names

"...errata to the specification published in early 2001 made underscores legal for the first time."

The article linked above says never use them, then gives a list of browsers that don't support them, all of which are, in terms of numbers of users at least, long-redundant.

I’ve answered your question in-depth here: http://mathiasbynens.be/notes/css-escapes

The article also explains how to escape any character in CSS (and JavaScript), and I made a handy tool for this as well. From that page:

If you were to give an element an ID value of ~!@$%^&*()_+-=,./';:"?><[]{}|`#, the selector would look like this:

CSS:

<style> #\~\!\@\$\%\^\&\*\(\)\_\+-\=\,\.\/\'\;\:\"\?\>\<\[\]\\\{\}\|\`\# { background: hotpink; } </style> 

JavaScript:

<script> // document.getElementById or similar document.getElementById('~!@$%^&*()_+-=,./\';:"?><[]\\{}|`#'); // document.querySelector or similar $('#\\~\\!\\@\\$\\%\\^\\&\\*\\(\\)\\_\\+-\\=\\,\\.\\/\\\'\\;\\:\\"\\?\\>\\<\\[\\]\\\\\\{\\}\\|\\`\\#'); </script> 

For those looking for a workaround, you can use an attribute selector, for instance, if your class begins with a number. Change:

.000000-8{background:url(../../images/common/000000-0.8.png);} /* DOESN'T WORK!! */ 

to this:

[class="000000-8"]{background:url(../../images/common/000000-0.8.png);} /* WORKS :) */ 

Also, if there are multiple classes, you will need to specify them in selector I think.

Sources:

  1. https://benfrain.com/when-and-where-you-can-use-numbers-in-id-and-class-names/
  2. Is there a workaround to make CSS classes with names that start with numbers valid?

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